Uncertainties in the proton lifetime

We discuss the masses of the leptoquark bosons m_x and the proton lifetime in grand unified theories based principally on SU(5). It is emphasized that estimates of m_x based on the QCD coupling and the fine structure constant are probably more reliable than those using the experimental value of sin² θ_w. Uncertainties in the QCD Λ parameter and the correct value of α are discussed. We estimate higher-order effects on the evolution of coupling constants in a momentum-space renormalization scheme. It is shown that increasing the number of generations of fermions beyond the minimal three increases m_x by almost a factor of 2 per generation. Additional uncertainties exist for each generation of technifermions that may exist. We discuss and discount the possibility that proton decay could be "Cabibbo rotated" away, and a speculation that Lorentz invariance may be violated in proton decay at a detectable level. We estimate that in the absence of any substantial new physics beyond that in the minimal SU(5) model the proton lifetime is 8 × 10^30±2 years.